Granddad was in his mid-90s when he died.
Reuters
On Sunday the team at Shedd Aquarium in Chicago had a tough decision to make.

Granddad the lungfish, who had lived at the aquarium since 1933 and was the oldest captive fish in the world, was in a state of rapidly declining health. Eventually, the team decided it was time to say goodbye.

According to the aquarium's website, Granddad was not just the oldest living resident there, but also the oldest fish in any public zoo or aquarium in the world, being in his mid-90s. In his final days, Granddad stopped eating and showed signs of organ failure, so the team decided the most humane thing to do was euthanize him.

President and CEO of the aquarium Dr Bridget Coughlin said in a statement: "It is incredible to know that more than 104 million guests had the opportunity to see Granddad in our care and learn about his unique species over eight decades," and that Granddad "sparked curiosity, excitement and wonder among guests of all ages."

Granddad apparently led an easy-going life, just as lungfish tend to do in the wild in their native Australia. They like to live in deep pools with free-slowing waters, but they get their name from possessing a primitive lung as well as gills, which means they can survive in stagnant water. However, they can't survive by breathing air alone.

In the wild, lungfish can live over 100 years. In Australia, they are a protected species. The fish have been around for nearly 400 million years, and fossils have shown they haven't changed much in over 100 million years.